Studies performed in out laboratory have established that a neuron specific form of the glycolytic enzyme enolase exists in brain which have been designated neuron specific enolase (NSE) and non-neuronal enolase or NNE. During the past year studies have focused on the developmental profile of NSE and on the further study of the presence of NSE in paraneuronal APUD cells. Neurons switch from NNE to NSE during development a process which has now been shown to be correlated to synaptogenesis. NSE is therefore not only a unique marker for neurons it also represents a useful probe for studying neuronal differentiation. The APUD cells of the diffuse neuroendocrine system have also been further studied with virtually all lung, gut, pituitary, and skin neuroendocrine cells staining for NSE. Neuroendocrine tumors (APUDomas) also contain high levels of NSE in contrast to non-endocrine tumors whih do not contain NSE. Clinical studies reveal highly elevated levels of NSE in oat cell carcinoma patients which decrease following chemotherapy indicating that NSE may be of diagnostic value.